--- /dev/null
+ <title>Video Output Overlay Interface</title>
+ <subtitle>Also known as On-Screen Display (OSD)</subtitle>
+
+ <para>Some video output devices can overlay a framebuffer image onto
+the outgoing video signal. Applications can set up such an overlay
+using this interface, which borrows structures and ioctls of the <link
+linkend="overlay">Video Overlay</link> interface.</para>
+
+ <para>The OSD function is accessible through the same character
+special file as the <link linkend="capture">Video Output</link> function.
+Note the default function of such a <filename>/dev/video</filename> device
+is video capturing or output. The OSD function is only available after
+calling the &VIDIOC-S-FMT; ioctl.</para>
+
+ <section>
+ <title>Querying Capabilities</title>
+
+ <para>Devices supporting the <wordasword>Video Output
+Overlay</wordasword> interface set the
+<constant>V4L2_CAP_VIDEO_OUTPUT_OVERLAY</constant> flag in the
+<structfield>capabilities</structfield> field of &v4l2-capability;
+returned by the &VIDIOC-QUERYCAP; ioctl.</para>
+ </section>
+
+ <section>
+ <title>Framebuffer</title>
+
+ <para>Contrary to the <wordasword>Video Overlay</wordasword>
+interface the framebuffer is normally implemented on the TV card and
+not the graphics card. On Linux it is accessible as a framebuffer
+device (<filename>/dev/fbN</filename>). Given a V4L2 device,
+applications can find the corresponding framebuffer device by calling
+the &VIDIOC-G-FBUF; ioctl. It returns, amongst other information, the
+physical address of the framebuffer in the
+<structfield>base</structfield> field of &v4l2-framebuffer;. The
+framebuffer device ioctl <constant>FBIOGET_FSCREENINFO</constant>
+returns the same address in the <structfield>smem_start</structfield>
+field of struct <structname>fb_fix_screeninfo</structname>. The
+<constant>FBIOGET_FSCREENINFO</constant> ioctl and struct
+<structname>fb_fix_screeninfo</structname> are defined in the
+<filename>linux/fb.h</filename> header file.</para>
+
+ <para>The width and height of the framebuffer depends on the
+current video standard. A V4L2 driver may reject attempts to change
+the video standard (or any other ioctl which would imply a framebuffer
+size change) with an &EBUSY; until all applications closed the
+framebuffer device.</para>
+
+ <example>
+ <title>Finding a framebuffer device for OSD</title>
+
+ <programlisting>
+#include <linux/fb.h>
+
+&v4l2-framebuffer; fbuf;
+unsigned int i;
+int fb_fd;
+
+if (-1 == ioctl(fd, VIDIOC_G_FBUF, &fbuf)) {
+ perror("VIDIOC_G_FBUF");
+ exit(EXIT_FAILURE);
+}
+
+for (i = 0; i < 30; i++) {
+ char dev_name[16];
+ struct fb_fix_screeninfo si;
+
+ snprintf(dev_name, sizeof(dev_name), "/dev/fb%u", i);
+
+ fb_fd = open(dev_name, O_RDWR);
+ if (-1 == fb_fd) {
+ switch (errno) {
+ case ENOENT: /* no such file */
+ case ENXIO: /* no driver */
+ continue;
+
+ default:
+ perror("open");
+ exit(EXIT_FAILURE);
+ }
+ }
+
+ if (0 == ioctl(fb_fd, FBIOGET_FSCREENINFO, &si)) {
+ if (si.smem_start == (unsigned long)fbuf.base)
+ break;
+ } else {
+ /* Apparently not a framebuffer device. */
+ }
+
+ close(fb_fd);
+ fb_fd = -1;
+}
+
+/* fb_fd is the file descriptor of the framebuffer device
+ for the video output overlay, or -1 if no device was found. */
+</programlisting>
+ </example>
+ </section>
+
+ <section>
+ <title>Overlay Window and Scaling</title>
+
+ <para>The overlay is controlled by source and target rectangles.
+The source rectangle selects a subsection of the framebuffer image to
+be overlaid, the target rectangle an area in the outgoing video signal
+where the image will appear. Drivers may or may not support scaling,
+and arbitrary sizes and positions of these rectangles. Further drivers
+may support any (or none) of the clipping/blending methods defined for
+the <link linkend="overlay">Video Overlay</link> interface.</para>
+
+ <para>A &v4l2-window; defines the size of the source rectangle,
+its position in the framebuffer and the clipping/blending method to be
+used for the overlay. To get the current parameters applications set
+the <structfield>type</structfield> field of a &v4l2-format; to
+<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant> and call the
+&VIDIOC-G-FMT; ioctl. The driver fills the
+<structname>v4l2_window</structname> substructure named
+<structfield>win</structfield>. It is not possible to retrieve a
+previously programmed clipping list or bitmap.</para>
+
+ <para>To program the source rectangle applications set the
+<structfield>type</structfield> field of a &v4l2-format; to
+<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY</constant>, initialize
+the <structfield>win</structfield> substructure and call the
+&VIDIOC-S-FMT; ioctl. The driver adjusts the parameters against
+hardware limits and returns the actual parameters as
+<constant>VIDIOC_G_FMT</constant> does. Like
+<constant>VIDIOC_S_FMT</constant>, the &VIDIOC-TRY-FMT; ioctl can be
+used to learn about driver capabilities without actually changing
+driver state. Unlike <constant>VIDIOC_S_FMT</constant> this also works
+after the overlay has been enabled.</para>
+
+ <para>A &v4l2-crop; defines the size and position of the target
+rectangle. The scaling factor of the overlay is implied by the width
+and height given in &v4l2-window; and &v4l2-crop;. The cropping API
+applies to <wordasword>Video Output</wordasword> and <wordasword>Video
+Output Overlay</wordasword> devices in the same way as to
+<wordasword>Video Capture</wordasword> and <wordasword>Video
+Overlay</wordasword> devices, merely reversing the direction of the
+data flow. For more information see <xref linkend="crop" />.</para>
+ </section>
+
+ <section>
+ <title>Enabling Overlay</title>
+
+ <para>There is no V4L2 ioctl to enable or disable the overlay,
+however the framebuffer interface of the driver may support the
+<constant>FBIOBLANK</constant> ioctl.</para>
+ </section>
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